HF is an extremely toxic and corrosive substance that is manufactured on a large scale for a range of industrial chemical processes. For example, HF is used extensively in the nuclear fuels industry for the hydro fluorination of uranium dioxide (UO2) to afford uranium tetrafluoride, required as a pre-enrichment feedstock for UF6 production. Acid fluorides are used on a small scale in chemical research and fine chemical products and are also constituents of some chemical weapons agents (CWAs), for example the military acetyl-choline esterase inhibitors (nerve agents) Sarin (GB, methylphosphonofluoridic acid 1-methylethylester), Soman (GD, methylphosphonofluoridic, 1,2,2-trimethylpropyl ester) and cyclohexylsarin (GF, methylphosphonofluoridic, cyclohexylester). Sarin is fatal in minute doses (0.01 mg of Sarin and per kg of body weight), is readily manufactured from basic starting materials with minimal apparatus and knowledge, and thus represent a credible terrorist threat. For example, 19 fatalities were caused by terrorist Sarin gas attacks on the Tokyo underground in 1994 and 1995.
In addition, some toxic industrial chemicals (TICs) contain fluorine atoms and can liberate HF or fluoride ions, for example in solution or in the gas phase.
Furthermore compounds which contain the cyanide ion tend to have the potential to release toxic hydrogen cyanide (HCN) under appropriate conditions. For example, the chemical warfare nerve agent Tabun (GA) releases HCN on hydrolysis under appropriate pH conditions.
Therefore, the detection of both fluoride and cyanide ions is thought to be important in environmental and industrial monitoring and for security and military purposes to detect certain chemical warfare agents.
Existing technology used, for example by UK armed forces and airport security systems, to detect noxious airborne agents, such as HF or HCN, is based upon ion mobility spectrometry, in which the diffusion of airborne contaminants is measured after ionisation with a radioactive source (usually 63Ni). Current devices may be portable IMS (ion mobility spectroscopy) devices including the man-portable chemical agent detector (MCAD), the lightweight chemical agent detector (LCAD) and the handheld chemical agent monitor (CAM (RTM)), all of which are manufactured by Smiths Detection. These devices are effective in detecting low levels of airborne agents, such as HF or HCN, but are susceptible to false positives and, due to the sophisticated nature of the technology involved, are fragile, and hence costly due to the persistent need to replace components. Furthermore, this fragility means that these devices cannot readily be used in situations in which they would be required to withstand rough handling (e.g. many military operations). Thus there is a need for simple robust technology to categorically identify these dangerous agents.
It is important to note that there are no innocuous vaporous sources of fluoride or cyanide and thus the ability to detect such materials is of industrial, civilian and military importance. Furthermore, because ambient levels of hydrogen fluoride and hydrogen cyanide are negligible, any detection of gaseous hydrogen fluoride, hydrogen cyanide or an airborne material that may release or be hydrolysed to hydrogen fluoride or hydrogen cyanide gas or the fluoride or cyanide anion (F− or CN−) can lead to a definitive identification of the presence of one of these dangerous agents.
Organometallic compounds that selectively bind fluoride ions in gaseous or aqueous form are known. For example, organometallic compounds based on ferrocene boronic esters have been shown to selectively bind fluoride anions with a change in the redox properties of the compound on fluoride binding [e.g., “Fluoride anion binding by cyclic boronic esters: influence of backbone chelate on receptor integrity”, C. Bresner et al., Dalton Trans., 2006, 3660-3667; “Multidentate Lewis acids: synthesis, structure and mode of action of a redox-based fluoride ion sensor”, S. Aldridge et al., Chem. Commun., 2002, 740-741; “Selective Electrochemical Detection of Hydrogen Fluoride by Ambiphilic Ferrocene Derivatives”, C. Bresner et al., Angew. Chem. Int. Ed. 2005, 44, 3606-3609; “Hydrogen-bonding motifs in the solid-state structure of ferroceneboronic acid”, C. Bresner et al., Acta Cryst. (2004). E60, m441-m443].
In addition a number of systems containing either an array of hydrogen-bond donors or appropriately spaced transition metal centres are known to be usable for detection of cyanide (Ahlers et al., Angew. Chem. Int. Ed. Eng. 1996, 35, 2141-2143) and the binding of cyanide by three-coordinate boranes is also known (R. Badugu et al. J. Am. Chem. Soc., 2005, 127, 3635-3641), even offering the possibility for sequestering cyanide from aqueous solution. Furthermore, the groups of both Jäkle and Gabbai have demonstrated the use of Lewis acid receptors containing the BMes2 (Mes=2,4,6-Me3C6H2) function to detect cyanide, in one case offering selective detection in the presence of fluoride in aqueous solution (T. W. Hudnall and F. P. Gabbai, J. Am. Chem. Soc., 2007, 129, 11978-11986).